It’s time to replace the fleet and at about $3.1 million per car, the transit agency will need to come up with about $2.1 billion to do it, Bay Area Rapid Transit staff told agency directors Thursday.

If cars are breaking down, the trains are late. If the trains are late, people won’t ride the system, said agency spokesman Linton Johnson.

“This is about being on time,” he said. “If you can’t rely on BART, then why ride.”

Currently, BART has about a 95 percent passenger on-time record.

But that could drop to say 80 percent or worse if the cars continue to decline over the next decade.

Thursday’s briefing is only the beginning of what will be a long process to design and buy the new cars, Linton said.

Commuters won’t see the new cars for at least another six years or so.

The new $3.1 million cars will have to fit the unique width in between the rails, requiring a custom design. Officials are hoping to add more space for strollers, wheelchairs, bikes and standing customers.

Staff is also floating the idea of three doors on each car instead of two.

As always, the big question will be how to pay for the purchase.

Back in 1971, the 669 trains costa total of $250 million, funded by the property taxes that built the system.

The Metropolitan Transportation Commission is looking to set aside about $20 million to help pay for the upgrades, but that pot of money won’t cover all the costs. BART will need a bond, a rate increase, extra state/federal funding, increased revenue or a combination of the above to pay the bill. But that decision is a ways down the road, Linton said.

“We’re looking at all options,” Linton said of the funding. “Now they’re going to be putting together final plans on what rail cars will need and the government affairs side is looking how to fund it.”

Researchers said Sunday the mass die-off occurred because unusually large amounts of sea ice forced penguin parents to travel farther in search of food for their young. By the time they returned, only two out of thousands of chicks had survived.